Chapter 12Inheritance Patterns and Human Genetics

Walter Sutton - American Geneticist

Developed the Chromosome Theory of Heredity

The Chromosome Theory of HeredityChromosomes are located in the nucleusFactors (genes) are found on chromosomesSutton discovered that genes are on chromosomes in 1902

Chromosome Theory of HeredityStates that genes are located on chromosomes and each gene occupies a specific place on a chromosomeOnly one allele is on a chromosomeFinally explains what Mendel was talking about

Thomas Hunt Morgan

Studied fruit flies Drosophila melanogaster

Fruit Flies are excellent for genetic studies because:

Reproduce quicklyEasy to raiseMany mutationsHave 8 chromosomes (n=4)

Morgan looked at TWO traitsGray bodies GNormal Wings - WBlack bodies gSmall wings w

P1 GGWWxggwwF1GgWw100%

Morgan then mated the F1 back to the recessive parentGgWw x ggwwExpected ratio 1:1:1:125% GgWw 25% Ggww25% ggWw 25% ggww

Morgans Actual Results41.5% gray normal41.5% black small 8.5 % black normal 8.5% gray small

ConclusionGene for body size and wing color were somehow connected or linkedCant undergo independent assortment

Gene LinkageGenes on the same chromosome are linked togetherInherited together THEREFORE they do not undergo independent assortment

Linkage GroupsPackage of genes that are always inherited togetherGenes on the same chromosomeOne linkage group for each homologous pairFruit flies 4 linkage groupsHumans 23 linkage groupsCorn 10 linkage groups

So linkage groups explain the high percentages (41.5%) butWhat about the 8.5%??????

17% had new combinationsThe combinations that were expected would be: Gray normal GWorBlack small - gw

When they are lined up they can become twisted and switch genesCrossing Over

The 17% that had new combinations are known asRecombinants individuals with new combinations of genesCrossing Over gives rise to new combinations Prophase I

Chromosome MapsThe likelihood a crossover will occur that will result in the separation of two genes depends on the distance between the two genesChromosome maps are diagrams that show the location of genes on a chromosomeTwo genes separated by crossing-over 1% of the time = one map unit apart Alfred Sturtevant (Morgans student) constructed the 1st chromosome map of fruit flies

NETTIE STEVENSWORKED WITH MEAL WORMSDISCOVERED THAT THEY HAD 20 CHROMOS. MALES HAD 19 REG. SIZE AND 1 SMALLSHE FOUND THE SAME THING IN FRUIT FLIESSHE SAID THE 19 THAT WERE THE SAME WERE AUTOSOMES, & THE MISMATCHED WERE SEX CHROMOSMALES HAVE 1 X AND 1 Y

Sex ChromosomesNettie Stevens made observations of meal worm chromosomes

Sex ChromosomesOne pairFemale XX

Male XY

Sex Determination 50/50

Genes on Sex ChromosomesSex chromosomes determine a persons sexSex chromosomes also contain other genes

Sex LinkedA gene located on a sex chromosomeUsually XExample Fruit Fly Eye ColorThe gene for eye color is on the X chromosome - not the Y

Fruit Fly Sex ChromosomesXXXY

FemalesXRXR

XRXr

XrXrMalesXRY

XrYRed EyedWhite Eyed

Sex-Linked Genetic DisordersGene for the trait is on the X or Y

X has many genes Y has few

Defects easy to spot appear more in males

CarrierA heterozygous female has the gene but does not express it can pass it on to her children

ColorblindnessRecessive X linked disorderCannot distinguish colorsDominant Gene XCRecessive Gene - Xc

HemophiliaRecessive X linked disorderBlood does not clotXH good geneXh hemophilia gene

Muscular DystrophyResults in the progressive wasting away of muscle

MutationsA change in the DNA of an organism

Can involve an entire chromosome or a single DNA nucleotide

May take place in any cell

MutationsGerm Cell Mutations - Occur in an organisms germ cells (gametes)- only affect offspring

Somatic Mutations - Take place in an organisms body cells and only affect the organism

MutationsLethal Mutation: Can cause death Often before birthGood Mutations:organisms have a better chance to reproducehave an evolutionary advantageProvide the variation on which natural selection acts

Chromosome MutationsChanges in the structure of a chromosomeLoss or addition of an entire chromosomeFour Types:duplicationdeletioninversiontranslocation

Gene Mutations

Point mutation-single nitrogen base is changedSubstitutions may not be fatal - there is redundancy in the amino acid codons

Frameshift MutationWhen a nucleotide is lost or added so that the remaining codons are grouped incorrectlyThis can code for the wrong amino acid and create an incorrect proteinInsertions and deletions are frameshift mutations

THE FAT CAT ATE THE RAT

Deletion causes a frameshift:If you delete the E in the

THF ATC ATA TET HER AT

NondisjunctionSome chromosome mutations alter the number of chromosomes found in a cellNondisjunction the failure of a chromosome to separate from its homologue during meiosis

NondisjunctionExample: Trisomy 21 Nondisjunction also affects sex chromosomesExamples: Klinefelters and Turners syndrome

PolyploidyCondition in which an organism has an extra set of chromosomes3N, 4NUsually fatal in animalsPlants usually more robustCaused by - Nondisjunction

Mutationshttp://learn.genetics.utah.edu/content/begin/traits/predictdisorder/

Pedigree ChartA family record that shows how a trait is inherited over several generations.Circles are females, squares are malesBlank = NormalHalf = Carrier not shown, pass onFilled = Displays trait

Pedigree chartFemale circleColored - recessive Male squareEmpty - dominant

Genetic Traits and DisordersGenetic disorders diseases or debilitating condition that has a genetic basisSingle Allele TraitsControlled by a signal alleleUsually recessiveExample: Hitchhikers thumbMultiple Allele TraitsControlled by 3 or more allelesExample: Blood typesPolygenic TraitsA trait that is controlled by two or more genesExample: Skin color

Huntingtons DiseaseCaused by a single dominant alleleAppears in 30s or 40sProgressive loss of muscle control and mental function deathGene on chromosome 4

Sickle Cell AnemiaCaused by a change in one of the polypeptides found in hemoglobin (carries oxygen in RBC)One nucleotide differenceCodominant inheritance pattern HAHSHA - normal alleleHS Sickle Cell allele

Sickle Cell AnemiaSickle Cell Anemia common in people of African ancestry and from tropical regionsCarriers (heterozygous) of Sickle Cell trait (HAHS) resistant to malaria

Human Blood GroupsMultiple alleles genes with more than two formsExample ABO and Rh blood groups

* Remember an organism can have two alleles only!

Blood TypeBlood Type determined by the presence or absence of certain things in the bloodRBC (Red blood cell) can carry two different antigensAntigens molecules that can be recognized by the immune system

Genotypes and Phenotypesii

IAIA or IAi

IBIB or IBi

IAIBType O

Type A

Type B

Type AB

II. BLOOD GROUPS-TYPING

DONORS RECEIVERSAS CAN GIVE TO AS, ABS

BS CAN GIVE TO BS, ABS

ABS CAN GIVE TO ABS

OS CAN GIVE TO ANYONE!

THEY ARE A UNIVERSAL DONORAS CAN RECEIVE FROM AS & OS

BS CAN RECEIVE FROM BS & OS

OS CAN RECEIVE FROM OS ONLY!

ABS CAN RECEIVE FROM ABS, AS, BS, & OS (UNIVERSAL RECIPIENTS)

Rh Blood GroupsRh antigen also on RBC

Rh+ - have antigen (Dominant)Rh_ - no antigen (Recessive)

IF A PREGNANT WOMAN HAS AN Rh+ BABY & SHE IS Rh- THEN THIS COULD BE A PROBLEM IF THE CORD BLOOD MIXES W/THE MOMS

SHE COULD HAVE ANTIBODIES AGAINST THE BABIES BLOOD AND CAUSE DEATH TO THE BABY

Blood Typing Activityhttp://nobelprize.org/educational_games/medicine/landsteiner/

Polygenic InheritanceTraits controlled by two or more genesExamples height, skin color, animal coat patternsPhenotypes are seen in a range

POLYGENIC INHERITANCEINHERITANCE OF SEVERAL GENES TO EXPRESS A SINGLE TRAITLIKE SKIN COLOR

Sex Influenced TraitsA trait that is caused by a gene whose expression differs in males and femalesBaldness:A sex influenced traitSingle gene two allelesGene expression is influenced by male hormones

Baldness

GenotypeMaleFemaleBBNormalNormalbbBaldBaldBbBaldNormal

Diagnosis of Genetic DisordersDowns Syndrome Trisomy 21Extra copy of the 21st chromosomeCan be detected by microscopic examinations of chromosomes karyotype

Prenatal DiagnosisAmniocentesis removes fluid from the sac around the babyThe fluid can be used to grow cells and make a karyotypeChorionic Villus Biopsy cells are removed from the embryoFaster results than amnio

Prenatal DiagnosisCVB and Amnio make it possible to detect chromosomal abnormalitiesTest for: biochemical abnormalitiesPresence of certain DNA sequences

EthicsWe can detect over 100 disordersKnowledge leads to choices and decisionsEthical Considerations:Do you consider a developing embryo a person? Are you pro-life or pro-choice?Should a person knowingly bring a baby with a deadly disease into the world?